EMI/RFI and vibration resistant electronics enclosure

ABSTRACT

A panel used in an enclosure for housing at least one electronic component, and an enclosure made of at least one of such panels is disclosed. The panel includes an electrically conductive layer, a separate, magnetically permeable layer connected to the electrically conductive layer, and a separate, extensionally damped layer covering at least 80 percent of a surface of the panel and attached to one or more of the electrically conductive layer and the magnetically permeable layer, where electrical and magnetic conductivity are maintained between all panels of the enclosure, and a ground is provided between circuitry of the electrical component and the electrically conductive layer.

BACKGROUND OF THE INVENTION

[0001] Sensitive electronics are often prone to radio frequencyinterference (RFI) as well as electromagnetic interference (EMI) atfrequencies ranging up to several megahertz. The electrical performanceof an analog or digital circuit can be impaired significantly by suchtypes of interference. In addition, vibrational energy impinging upon anenclosure containing such circuits and transmitted into the electroniccomponents themselves can impair circuit performance to a degreedependent upon the microphonic sensitivity of the electronic componentsand the internal wiring used. Sources of the RFI, EMI, and vibrationalenergy are assumed to be external to the enclosure and may include otherhigh frequency electronics (especially digital circuitry), electricalmotors, transformers, AC power lines (and cables), electrical switches,electrical lighting, loudspeaker output, etc. In addition to preventingRF, magnetic and vibrational energy from entering the circuitrycontained within the enclosure, it is desirable to prevent any RF ormagnetic energy generated by the contained circuitry from leaving theenclosure, where it could affect other sensitive electrical circuits.

[0002] Electronics are usually housed within an enclosure formed ofeither aluminum, steel, or plastic. Plastic enclosures offer noprotection from RFI or EMI unless they are sprayed with a highlyconductive coating (usually a metal) and properly grounded. This canprovide a degree of RFI protection, but offers very little protectionfrom magnetic interference, except at very high frequencies (>100 kHz).Steel enclosures offer some protection from magnetically generatedinterference, but sacrifice a significant degree of RFI protection dueto the fairly poor electrical conductivity of steel at very highfrequencies. This can be remedied somewhat by plating the steel with avery conductive metal such as copper. However, steel is still notentirely effective in preventing low frequency (e.g., 60 Hz) magneticinterference, especially if the field strength is high, because of themarginal magnetic permeability of the steel materials typically used. Analuminum enclosure offers good protection from RFI and very highfrequency EMI (assuming correct grounding guidelines are followed) dueto its high degree of electrical conductivity. Aluminum enclosures,however, offer very little protection from EMI at frequencies below 100kHz since they are not magnetically permeable, i.e., they aretransparent to low frequency magnetic fields. It is apparent that noneof the commonly used materials for electronics enclosures offers a highdegree of both RFI and EMI protection, particularly EMI protection atlow frequencies. In addition, immunity from vibrational energy is rarelyaddressed correctly in most enclosure designs. Often, materials withdubious damping qualities are used. In most cases, even when theappropriate materials are utilized, the surface area coverage isinadequate to provide any significant amount of vibrational energydamping, particularly at lower frequencies, below 200 Hz. It should beemphasized that usually, immunity from vibrational energy is notaddressed at all.

[0003] It is therefore an object of the present invention to prevent RF,magnetic and vibrational energy from entering the circuitry containedwithin an enclosure. It is a further object of the invention to providean enclosure that prevents entry of such energy, while preventing any RFor magnetic energy generated by the contained circuitry from leaving theenclosure, where it could affect other sensitive electrical circuits. Itis yet another object of the invention to provide an enclosure that doesnot have the detrimental qualities of enclosures formed solely of eitheraluminum, steel, or plastic.

SUMMARY OF THE INVENTION

[0004] The above objects and others are accomplished by a panel for anenclosure for housing at least one electronic component, which includesan electrically conductive layer, a separate, magnetically permeablelayer connected to the electrically conductive layer, and a separate,extensionally damped layer covering at least 80 percent of a surface ofthe panel and attached to one or more of the electrically conductivelayer and the magnetically permeable layer.

[0005] The electrically conductive layer may be formed of a materialconductive at frequencies greater than 100 kHz and include at least oneof aluminum, copper-plated steel, plastic which is sprayed with aconductive coating, and plastic which is vapor coated with a conductivecoating.

[0006] The magnetically permeable layer comprises at least one ofCO-NETIC® and NETIC® magnetic shielding alloy and is a foil of at leastabout 0.1 mm in thickness, preferably at least about 0.25 mm. TheCO-NETIC® magnetic shielding alloy can be CO-NETIC AA foil, and saidNETIC® magnetic shielding alloy can be NETIC S3-6 foil. The magneticshielding alloy should be able to protect the housed components from lowfrequency EMI, specifically that below 100 kHz, and magneticinterference below 60 Hz. The magnetic shielding should be hydrogenannealed, especially when it will be subject to bending and otherforming.

[0007] The extensionally damped layer may include a single passextruded, thermoplastic alloy damping material, such as ISODAMP® CN,with a thickness of at least about 1.5 mm. Preferably, the thickness ofthe ISODAMP® CN layer is at least about 3.1 mm. The extensionally dampedlayer may also be Sorbothane®, with a thickness of at least about 3.2mm. The extensionally damped layer is able to protect the housedcomponents from vibrational energy at high and low frequencies,especially very low frequencies below 200 Hz.

[0008] Each of the electrically conductive layer, magnetically permeablelayer, and extensionally damped layer are preferably connected using anadhesive material.

[0009] The above objects and others are further accomplished by anenclosure for housing at least one electronic component, which comprisesat least one panel having properties as detailed above.

[0010] The electrically conductive layer is preferably an outermostlayer, and the extensionally damped layer is preferably an innermostlayer. Electrical and magnetic conductivity must be maintained betweenall panels of the enclosure, and a ground is provided between circuitryof the electrical component and the electrically conductive layer. Aconnector, which connects each of the panels together, can providecontinuity of electrical conductivity throughout the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows the elements of a typical panel prepared according tothe present invention.

[0012]FIG. 2 shows an enclosure made of panels such as that shown inFIG. 1, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] The above objects and others are accomplished by the multi-layerenclosure 20 shown in FIG. 2, the layers in each panel of which aredescribed in detail below. A typical panel of the enclosure is shown inFIG. 1. The electrically conductive layer 11 of the composite panel 10should be formed of a material having a high degree of electricalconductivity, especially at very high frequencies (above 100 kHz). In apreferred embodiment of the invention, the electrically conductive layer11 comprises aluminum. Other materials that may be used as theelectrically conductive layer 11 include copper-plated steel and plasticwhich is sprayed or vapor-deposited with a highly conductive coating.The electrically conductive layer 11 is typically the outer layer in apanel 10, although it may be surrounded by another suitable layer. Whenthe electrically conductive layer 11 is the outermost layer, it servesboth cosmetic and structural purposes, as it is the visible portion andprovides mechanical rigidity and form.

[0014] Processing of the electrically conductive layer 11 by painting oranodizing should not have a detrimental effect, provided that electricalcontinuity between the panels 10 is not compromised. Since jointsbetween each panel 10 must maintain a high degree of electricalconductivity, it may be necessary to mask off critical joint areas orprovide appropriate secondary processing to ensure good conductivity ifpainting or anodizing is employed.

[0015] The magnetically permeable layer 12 of the composite panel 10should be formed of a material which exhibits a very high degree ofmagnetic permeability as well as a moderately high magnetic saturationlimit. In a preferred embodiment of the invention, the magneticallypermeable layer 12 comprises CO-NETIC® and/or NETIC® magnetic shieldingalloys manufactured by Magnetic Shield Corporation (Perfection MicaCo.). The exact alloy and thickness utilized will depend upon theexpected strength of the impinging H-field and the degree of shielding(dB of attenuation) required. In most audio electronic enclosures, forexample, a foil of 0.1 mm (0.004 in.) in thickness is usually adequate.For critical applications, a foil of 0.25 mm (0.010 in.) in thicknessshould be used. The “CO-NETIC AA” and “NETIC S3-6” foils are availableat varying thickness and are perfection (hydrogen) annealed. They arealso available with a pressure sensitive tape backing to facilitateattachment to the electrically conductive or structural layer 11.Detailed specifications for these materials are provided in MagneticShielding Material and Fabrication Guide, Catalog MG-6, 3-7, 1997,published by Magnetic Shield Corporation, which is incorporated hereinby reference. These materials are highly corrosion resistant and do notrequire any type of painting or surface protection.

[0016] Conventional shearing, blanking, stamping, forming and bendingoperations are acceptable for fabrication. It is important, however,that severe forming or drawing should be avoided, as the magneticshielding properties can be degraded. Stress annealed sheets of thesealloys are available in thicker stock when higher magnetic permeabilityis necessary or more severe forming is required. The stress annealedsheet materials would require hydrogen annealing following fabricationto attain maximum magnetic shielding characteristics.

[0017] The extensionally damped layer 13 should be formed of a materialpossessing excellent damping characteristics, i.e., very high losses. Ina preferred embodiment of the invention, the extensionally damped layer13 comprises a single pass extruded, high temperature fusedthermoplastic alloy damping material produced by EAR SpecialtyComposites, such as ISODAMP® CN. The properties of ISODAMP® CN aredescribed in Technical Data Sheet TDS-47, published by EAR SpecialtyComposites, which is incorporated herein by reference. Materialthickness should be a minimum of about 1.5 mm (0.060 in.). In a mostpreferred embodiment of the invention when ISODAMP® CN is used, thematerial thickness is about 3.1 mm (0.122 in.).

[0018] A pressure sensitive adhesive or similar backing is available tofacilitate attachment to the other layers of the composite panels 10. Itis crucial that at least 80% of the surface area of each enclosure panel10 be covered by the extensionally damped layer 13 in order to becompletely effective. Typically, this layer 13 would be formed bydie-cutting individual pieces and then attaching them to the interiorsurfaces of each panel 10 of the enclosure.

[0019] In another preferred embodiment of the invention, a material forthis layer is Sorbothane®, a thermoset, polyether-based damping materialmanufactured by Sorbothane Inc. The properties of Sorbothane® areprovided in Sorbothane, the Ultimate Damping Material, published bySorbothane Inc., which is incorporated herein by reference. In a mostpreferred embodiment of the invention when Sorbothane® is used, theextensionally damped layer 13 is about 3.2 mm thick, with a durometer 30hardness and a PSA backing.

[0020] Implementation of the multi-layer panel 10 is fairlystraightforward and similar to standard electronic enclosure designsexcept for the multiple layer composite panel construction and severalkey requirements regarding panel assembly. As previously mentioned, itis critical that the following requirements be met after assembly of theenclosure:

[0021] 1. Electrical conductivity must be maintained between all panelsof the enclosure. In addition, a high quality ground must be providedbetween the internal circuitry and the electrically conductive layers 11of the enclosure. Good RF design guidelines should also be followed.

[0022] 2. Magnetic conductivity, i.e., low reluctance, must bemaintained between all panels 10 of the enclosure.

[0023] 3. At least 80% of the surface area of the inner surface of allpanels of the enclosure must be covered by the extensional damping layer13.

[0024] The typical enclosure 20 assembly detail in FIG. 2 meets theabove requirements. The magnetically permeable foil 12 is attached,using glue or PSA, to the inside of the outer, electrically conductivelayer 11, which also serves as a structural layer. The magneticallypermeable foil 12 is wrapped around the edge of the upper flange 15 ofthe side or rear panel 17. When the top panel 16 is attached, magneticcontact is maintained at magnetic contact point 18 between the foillayers 12 of the two panels 16, 17. Similarly, electrical contact ismade at electrical contact point 19 when the two panels 16, 17 arescrewed together. A conductive screw 14, e.g., copper, helps to ensureelectrical continuity. Extensional damping material 13 is applied toeach panel in a secondary operation, before final assembly of thepanels. All types of enclosure assembly designs can be accommodated,e.g., two three-sided, U-shaped sections or separate panels attached toa sub-frame. The standard cosmetic front panel 16 could easily beattached afterwards. The exact panel design, attachment method andoverall assembly are entirely up to the needs and discretion of thedesigner.

[0025] The above-described invention can be utilized to form either anoverall enclosure which houses multiple components and circuitry, or, toform housings for individual electrical or electronic components whichmay be prone to either external source interference or are actuallygenerators of such interference. In either case, the concepts ofinsulation are the same, although in the latter case individual shieldsmust be formed. An overall enclosure for multiple components andcircuitry may be used when cost savings is desired. It is also possibleto use the above-described invention to isolate a single component orcircuit from other components and circuitry housed within the sameenclosure in ultra-critical applications. Examples include power orsignal transformers, sensitive ADC or DAC circuitry, phono-circuitry,digital clocks, microphone amplifier circuitry, etc. It will benecessary to determine the exact material specifications, i.e.,composition and thickness of each layer, based upon the degree ofexpected interference, e.g., external H-field strength, the degree ofdesired isolation, ease of fabrication, cost, etc. As the position ororder of each layer within the composite panel is not critical, it ispossible to alter the design based upon the needs of a particularapplication. For example, an individual transformer housing can beformed with the extensional damping material attached to the outside ofthe electrically conductive layer, while the magnetically permeable foilis still attached to the inside surface.

[0026] Having described an embodiment of the invention, it is to beunderstood that the invention is not limited to any of the preciseembodiments described herein. Various changes and modifications could beeffected by one skilled in the art without departing from the spirit orscope of the invention as defined in the appended claims.

What is claimed is:
 1. A panel for an enclosure for housing at least oneelectronic component, which comprises: an electrically conductive layer;a separate, magnetically permeable layer, connected to said electricallyconductive layer; and a separate, extensionally damping layer, coveringat least 80 percent of a surface of said panel, and attached to at leastone of said electrically conductive layer and said magneticallypermeable layer.
 2. A panel as set forth in claim 1 , wherein saidelectrically conductive layer is formed of a material conductive atfrequencies greater than 100 kHz.
 3. A panel as set forth in claim 2 ,wherein said electrically conductive layer comprises at least one ofaluminum, copper-plated steel, plastic which is sprayed with aconductive coating, and plastic which is vapor coated with a conductivecoating.
 4. A panel as set forth in claim 1 , wherein said magneticallypermeable layer comprises at least one of CONETIC® and NETIC® magneticshielding alloy.
 5. A panel as set forth in claim 1 wherein saidmagnetically permeable layer is a foil of at least about 0.1 mm inthickness.
 6. A panel as set forth in claim 5 , wherein saidmagnetically permeable layer is a foil of at least about 0.25 mm inthickness.
 7. A panel as set forth in claim 4 , wherein said CONETIC®magnetic shielding alloy is CO-NETIC AA foil, and said NETIC® magneticshielding alloy is NETIC S3-6 foil, said magnetic shielding alloy beinghydrogen annealed.
 8. A panel as set forth in claim 1 , wherein saidextensionally damping layer comprises a single pass extruded,thermoplastic alloy damping material.
 9. A panel as set forth in claim 8, wherein said extensionally damping layer is ISODAMP® CN, with athickness of at least about 1.5 mm.
 10. A panel as set forth in claim 9, wherein said extensionally damping layer is of a thickness of at leastabout 3.1 mm.
 11. A panel as set forth in claim 8 , wherein saidextensionally damping layer is Sorbothane®, with a thickness of at leastabout 3.2 mm.
 12. A panel as set forth in claim 1 , wherein each of saidelectrically conductive layer, said magnetically permeable layer, andsaid extensionally damping layer are connected using an adhesivematerial.
 13. An enclosure for housing at least one electroniccomponent, which comprises at least one panel, wherein each of said atleast one panel comprises: an electrically conductive layer; a separate,magnetically permeable layer, connected to said electrically conductivelayer; and a separate, extensionally damping layer, covering at least 80percent of a surface of said panel, and attached to at least one of saidelectrically conductive layer and said magnetically permeable layer. 14.An enclosure as set forth in claim 13 , wherein said electricallyconductive layer is formed of a material conductive at frequenciesgreater than 100 kHz, and which comprises at least one of aluminum,copper-plated steel, plastic which is sprayed with a conductive coating,and plastic which is vapor coated with a conductive coating.
 15. Anenclosure as set forth in claim 13 , wherein said magnetically permeablelayer comprises at least one of CO-NETIC® and NETIC® magnetic shieldingalloy.
 16. An enclosure as set forth in claim 13 , wherein saidextensionally damping layer comprises a single pass extruded,thermoplastic alloy damping material which comprises at least one ofISODAMP® CN, and Sorbothane®.
 17. An enclosure as set forth in claim 13, wherein each of said electrically conductive layer, said magneticallypermeable layer, and said extensionally damping layer are connectedusing an adhesive material.
 18. An enclosure as set forth in claim 13 ,wherein said electrically conductive layer is an outermost layer, andsaid extensionally damping layer is an innermost layer.
 19. An enclosureas set forth in claim 13 , wherein electrical and magnetic conductivityare maintained between all panels of said enclosure, and a ground isprovided between circuitry of said electrical component and saidelectrically conductive layer.
 20. An enclosure as set forth in claim 13, wherein a connector, which connects each of said at least one paneltogether, provides continuity of electrical conductivity throughout saidenclosure.
 21. A method of making an enclosure for housing at least oneelectronic component, which comprises: providing at least one panel,which comprises: an electrically conductive layer, a separate,magnetically permeable layer, and a separate, extensionally dampinglayer; attaching said magnetically permeable layer to said electricallyconductive layer; attaching said extensionally damping layer to at leastone of said electrically conductive layer and said magneticallypermeable layer, whereby at least 80 percent of a surface of said panelis covered with said extensionally damping layer; and maintainingelectrical and magnetic conductivity between all of said at least onepanel of said enclosure.
 22. A method as set forth in claim 21 , furthercomprising: providing a ground between circuitry of said electricalcomponent and said electrically conductive layer.
 23. A method as setforth in claim 21 , wherein said maintaining electrical conductivitycomprises using an electrically conductive connector to connect each ofsaid at least one panel together.